Horizontal travelling grate pelletizing machines and certain types of grate kilns are widely used in the preparation of finely divided pelletizable materials, such as pelletized iron ore. A typical horizontal travelling grate pelletizing machine includes a travelling grate which consists of an array of a plurality of edge abutting slotted pallets that travel along a straight horizontal track to define a continuous material carrying grate with a gas permeable bottom. In a typical iron ore pelletizing operation moist spherical agglomerates of an iron ore filter cake with a nominal diameter of, for example, about one-half inch (termed "green balls") are deposited in a layer or bed of predetermined thickness at the charging end of the grate. The green balls are subjected to drying, high temperature heating, and cooling as the grate moves through several zones of the pelletizing machine. The green balls are completely heat hardened and cooled before being discharged from the grate as product. These treatments involve passing air in the cooling zone and hot gases in other zones through the bed of green balls on the grate, either in down draft or up draft or in a combination of down draft and up draft gas flows. Windboxes are usually positioned below the grate for receiving and transmitting the air and the hot gases.
A typical grate kiln type of pelletizing machine includes a straight travelling grate having a gas permeable bottom and upwardly extending confronting side walls, the grate usually being a chain grate. Green balls are deposited on the grate and travel on the grate through drying and preheating zones which partially harden the balls; the partially hardened balls are discharged from the grate into a rotary kiln that rotates about an axis that is inclined downwardly from its receiving end to its discharge end. The heat hardening is completed in the rotary kiln after which the balls pass through a cooler. Hot combustion gases are introduced into the rotary kiln at its discharge to complete the heat hardening of the balls as they tumble in the rotary kiln. Hot gases also pass one or more times through the balls on the grate in the drying and preheating operations. Windboxes are mounted below the grate for receiving and/or transmitting the hot gases.
Each of the foregoing types of equipment usually has an open area between the windboxes and the travelling grate at the charge and discharge ends of the grate. These open areas are a source of heat loss due to the penetration of external air or the escape of hot gases. With the ever increasing costs of fuel to operate equipment of this type it is essential that these open areas be sealed as well as practicable. Attempts have been made to seal these open areas. For example, fixed sealing plates or dead plates have been arranged adjacent the end windboxes with as little clearance as possible to the underside of the pallets passing over the sealing plate. However, with fixed sealing plates of this type the problem of jamming of the pallets thereon exists. In order to avoid jamming the plates have been designed to flex downwardly away from the pallet. A spring type mounting has been used to provide such flexing. However, such flexibly mounted sealing plates have not been entirely adequate in preventing penetration of excess air into or escape of gases from the end windboxes. In addition, the spring mounted plates have a tendency to move laterally with respect to the pallets and thereby open more excape areas for the gases. Another variation of such plates includes the use of beds of granular material maintained at an appropriate level by scraping action of the pallet cross members.
U.S. Pat. No. 3,713,634 describes a sealing device for use with travelling grates that includes two sealing plates arranged one behind the other in the direction of travel of the pallets of the travelling grate, each of the sealing plates extending over at least two pallet cross-supports. This structure includes a lever which is pivotally supported at an intermediate point thereof to the machine structure, one end of the lever being pivotally secured to the sealing plate. The lever is provided with a counterweight for urging the sealing plate in the direction of the pallets.
U.S. Pat. No. 3,719,354 discloses the use of dead plates extending over the width of a travelling grate at the positions wherein the pallets enter and leave the treatment zone of a grate kiln. Each of the pallets have wear strips depending from the underside of the leading and trailing ends thereof which contact the dead plates as they pass thereover. The wear strips are formed of a material that is softer than the dead plates, whereby the wear strips are worn away to accommodate for distortion of the pallets.
It would be advantageous to provide a sealing system for grate kilns and the like employing a travelling grate that is relatively simplified in design and construction and at the same time offers sufficient structural characteristics so as to provide enhanced savings in heat loss and significantly reduce the number of repairs of replacements required and, accordingly, reduce the down time of such grate kilns and the like.